This invention concerns novel laminates which can withstand high temperatures for extended periods of time, possess good low temperature impact strength, and have good resistance to permeation of fuel compositions, especially in automotive applications. The laminates disclosed in the present application contain at least two layers. One layer is a blow moldable thermoplastic polyamide composition, and the second layer is a polyvinyl alcohol (PVOH) or copolymer of vinyl alcohol.
Processes for preparing a blow moldable thermoplastic polyamide composition are known in the art. In addition, multi-layer articles of at least a permeation barrier resin, namely the PVOH or copolymer of vinyl alcohol, hereinafter referred to as vinyl alcohol compositions, and at least a polyolefin are also known. However, no art exists which discloses a multi-layer laminate of a vinyl alcohol composition and a thermoplastic blow moldable polyamide composition.
In addition, there is no prior art on laminates comprising at least one layer of a blow moldable thermoplastic polyamide composition which have good high temperature tensile properties, low temperature impact strength and good fuel permeation resistance. However, U.S. Pat. Nos. 4,243,724 and 4,243,074 disclose laminates made from at least two layers comprising a polyamide compound and a polyamide/polyvinyl alcohol. These references do not teach the use of a blow moldable thermoplastic polyamide or the use of at least one layer being a vinyl alcohol or copolymers of vinyl alcohol which are not further melt blended with a second polyamide.
Moreover, the present application discloses laminates which can be made into large containers, in particular, fuel tanks. In order to blow mold such large containers, excellent melt strength is required. Thus, the laminates disclosed in the present application incorporate a fibrillatable fluoropolymer to the thermoplastic polyamide composition in one of the layers in order to achieve the necessary melt strength in order for the laminate to be blow molded into a large container.
As such, the laminates disclosed in the present application are particularly suitable for fuel tanks for motor vehicles.
Currently, there is a market for plastic fuel tanks which can withstand high temperatures (130.degree. C.) for an extended period of time. The current plastic gas tank is generally made of high density polyethylene (HDPE), which melts at 125.degree. C., and requires expensive heat shielding to provide protection from high temperatures generated by the engine and exhaust components of motor vehicles. Much of the auto industry has committed to plastic gas tanks because of their improved safety, design flexibility, lower weight, lower corrosivity, and higher volume tanks in a given area compared to metal tanks.
A high temperature resistant fuel tank has the following characteristics: 1) ability to withstand continuous use exposure at 130.degree. C.; 2) low temperature impact strength, i.e. the tank must pass (no cracks) a 6 meter drop at -40.degree. C., filled with ethylene glycol; and 3) good fuel permeation resistance. In addition, the thermoplastic polymer compositions used to make these laminates must have good melt strength sufficient to blow mold tanks which typically range in size from 8 to 20 gallons.
HDPE is a well-known resin which is commonly used to make plastic fuel tanks. With respect to the characteristics described above, HDPE possesses adequate melt strength and low temperature, but it has poor fuel permeation resistance and cannot withstand temperatures of 130.degree. C. Polyamide resins are not known for use in fuel tanks. Some blow moldable polyamides have the required melt strength necessary to manufacture fuel tanks with good fuel barrier; however, they are marginal at continuous use exposure at high temperatures, and have very poor low temperature toughness. These blow moldable polyamides obtain their melt strength by i) having a very high molecular weight, or ii) epoxy cross-linking.
The addition of the fibrillatable fluoropolymer to enhance the melt strength of the thermoplastic polyamide composition is known in the art. However, this addition of fluoropolymer dramatically reduces toughness in the polyamide compositions, and the polyamide compositions are considered too brittle to be useful in their applications, such as fuel tanks.
Accordingly, the addition of fibrillatable fluoropolymer to the thermoplastic blow moldable polyamide compositions in the present invention does not reduce toughness significantly; the resins exhibit non-break notched Izod at -29.degree. C., and pass the -40.degree. C. drop test at 6 meters designed for fuel tanks.
Thus, the laminates of the present invention when made into large containers provide: 1) excellent melt strength to blow mold fuel tanks; 2) continuous use exposure at 130.degree. C.; 3) low temperature impact strength; and 4) good fuel permeation resistance. All four characteristics are required for high temperature fuel tanks.
No other laminates in the prior art can provide all of the above characteristics.